Ticks choose humans over dogs in hot weather, study finds

A type of tick which carries a potentially fatal disease is more likely to feed on humans than dogs when temperatures increase, a study has found, raising concerns about the impact of climate change on people’s health.

Scientists in the US made the findings after setting up a series of temperature-controlled experiments in which a human and a dog were put in separate wooden boxes, linked by a clear plastic tube lined with ticks.

This test allowed researchers to observe that the tropical lineage tick, a variety of “brown dog" tick, was two and a half times more likely to choose the human over the dog when the temperature rose from 23.3C to 37.8C.

These ticks carry the bacteria which causes the Rocky Mountain spotted fever (RMSF), an illness whose fatality rate can exceed one in five people if an infection is not spotted in the first week. Symptoms include headache, muscle aches and fever.

The number of cases of this and similar diseases, known as spotted fever rickettsiosis, have rocketed over the past two decades, according to the Centres for Disease Control and Prevention. 

Laura Backus, who led the study at the University of California, Davis School of Veterinary Medicine , said: "Our work indicates that when the weather gets hot, we should be much more vigilant for infections of RMSF in humans."

Although the tropical lineage tick is only currently found in southern parts of the US like Arizona, Ms Backus warned that its range is predicted to expand northwards with rising temperatures.

The researchers also discovered that temperate ticks were less inclined to choose dogs as temperatures rose.

"We believe that this decreased preference for dogs - combined with a slight increase in preference for humans - suggests that hot temperatures may also elevate risks of RMSF in areas where the temperate ticks are more common," Ms Backus said.

Joel Breman, president of the American Society of Tropical Medicine and Hygiene, where the research was presented on Monday, said the experiment “adds to the growing evidence of the increasing connection between climate change and its impact on health”.

"Climate change is moving so quickly that it is critical to keep pace with the many ways it may alter and intensify the risk of a wide range of infectious diseases so we are better prepared to diagnose, treat and prevent them,” he added.